[polly] r246441 - Always use the branch instructions to model the PHI-node writes

Mon Aug 31 06:44:51 PDT 2015

I don't believe this change was necessary because we have this part:

    Instruction *OpI = dyn_cast<Instruction>(Op);
    if (OpI) {
      BasicBlock *OpIBB = OpI->getParent();
      // As we pretend there is a use (or more precise a write) of OpI in OpBB
      // we have to insert a scalar dependence from the definition of OpI to
      // OpBB if the definition is not in OpBB.
      if (OpIBB != OpBB) {
        IRAccess ScalarRead(IRAccess::READ, OpI, ZeroOffset, 1, true, OpI);
        AccFuncMap[OpBB].push_back(std::make_pair(ScalarRead, PHI));
        IRAccess ScalarWrite(IRAccess::MUST_WRITE, OpI, ZeroOffset, 1, true,
                              OpI);
        AccFuncMap[OpIBB].push_back(std::make_pair(ScalarWrite, OpI));
      }
    }

Please tell me where I am mistaken?

On 08/31, Tobias Grosser via llvm-commits wrote:
> Author: grosser
> Date: Mon Aug 31 08:45:54 2015
> New Revision: 246441
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=246441&view=rev
> Log:
> Always use the branch instructions to model the PHI-node writes
> 
> Before this commit we did this only for Arguments or Constants, but indeed
> an instruction may define a value a lot higher up in the dominance tree, but
> the actual write generally needs to happen right before branching to the
> PHI node. Otherwise, the writes of different branches into PHI nodes may get
> intermixed if they lay higher up in the dominance tree.
> 
> Added:
>     polly/trunk/test/Isl/CodeGen/non-affine-phi-node-expansion-3.ll
> Modified:
>     polly/trunk/lib/Analysis/TempScopInfo.cpp
> 
> Modified: polly/trunk/lib/Analysis/TempScopInfo.cpp
> URL: http://llvm.org/viewvc/llvm-project/polly/trunk/lib/Analysis/TempScopInfo.cpp?rev=246441&r1=246440&r2=246441&view=diff
> ==============================================================================
> --- polly/trunk/lib/Analysis/TempScopInfo.cpp (original)
> +++ polly/trunk/lib/Analysis/TempScopInfo.cpp Mon Aug 31 08:45:54 2015
> @@ -128,10 +128,9 @@ void TempScopInfo::buildPHIAccesses(PHIN
>        }
>      }
>  
> -    // If the operand is a constant, global or argument we use the terminator
> -    // of the incoming basic block as the access instruction.
> -    if (!OpI)
> -      OpI = OpBB->getTerminator();
> +    // Always use the terminator of the incoming basic block as the access
> +    // instruction.
> +    OpI = OpBB->getTerminator();
>  
>      IRAccess ScalarAccess(IRAccess::MUST_WRITE, PHI, ZeroOffset, 1, true, Op,
>                            /* IsPHI */ true);
> 
> Added: polly/trunk/test/Isl/CodeGen/non-affine-phi-node-expansion-3.ll
> URL: http://llvm.org/viewvc/llvm-project/polly/trunk/test/Isl/CodeGen/non-affine-phi-node-expansion-3.ll?rev=246441&view=auto
> ==============================================================================
> --- polly/trunk/test/Isl/CodeGen/non-affine-phi-node-expansion-3.ll (added)
> +++ polly/trunk/test/Isl/CodeGen/non-affine-phi-node-expansion-3.ll Mon Aug 31 08:45:54 2015
> @@ -0,0 +1,49 @@
> +; RUN: opt %loadPolly -polly-codegen -polly-no-early-exit \
> +; RUN:     -polly-detect-unprofitable -S < %s | FileCheck %s
> +
> +define void @foo(float* %A, i1 %cond0, i1 %cond1) {
> +entry:
> +  br label %loop
> +
> +loop:
> +  %indvar = phi i64 [0, %entry], [%indvar.next, %backedge]
> +  %val0 = fadd float 1.0, 2.0
> +  %val1 = fadd float 1.0, 2.0
> +  %val2 = fadd float 1.0, 2.0
> +  br i1 %cond0, label %branch1, label %backedge
> +
> +; CHECK-LABEL: polly.stmt.loop:
> +; CHECK-NEXT:    %polly.subregion.iv = phi i32 [ 0, %polly.stmt.loop.entry ]
> +; CHECK-NEXT:    %p_val0 = fadd float 1.000000e+00, 2.000000e+00
> +; CHECK-NEXT:    %p_val1 = fadd float 1.000000e+00, 2.000000e+00
> +; CHECK-NEXT:    %p_val2 = fadd float 1.000000e+00, 2.000000e+00
> +; CHECK-NEXT:    store float %p_val0, float* %merge.phiops
> +; CHECK-NEXT:    store float %p_val1, float* %val1.s2a
> +; CHECK-NEXT:    store float %p_val2, float* %val2.s2a
> +
> +; FIXME -> The last two writes are not really needed and can be dropped if the
> +;          incoming block of the PHI and the value that is used share the same
> +;          non-affine region.
> +
> +branch1:
> +  br i1 %cond1, label %branch2, label %backedge
> +
> +; CHECK-LABEL: polly.stmt.branch1:
> +; CHECK-NEXT:    store float %p_val1, float* %merge.phiops
> +
> +branch2:
> +  br label %backedge
> +
> +; CHECK-LABEL: polly.stmt.branch2:
> +; CHECK-NEXT:    store float %p_val2, float* %merge.phiops
> +
> +backedge:
> +  %merge = phi float [%val0, %loop], [%val1, %branch1], [%val2, %branch2]
> +  %indvar.next = add i64 %indvar, 1
> +  store float %merge, float* %A
> +  %cmp = icmp sle i64 %indvar.next, 100
> +  br i1 %cmp, label %loop, label %exit
> +
> +exit:
> +  ret void
> +}
> 
> 
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-- 

Johannes Doerfert
Researcher / PhD Student

Compiler Design Lab (Prof. Hack)
Saarland University, Computer Science
Building E1.3, Room 4.31

Tel. +49 (0)681 302-57521 : doerfert at cs.uni-saarland.de
Fax. +49 (0)681 302-3065  : http://www.cdl.uni-saarland.de/people/doerfert
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